1. Field of the Invention
This invention relates to correction of convergence in color television sets, such as television monitors and projector-type television sets, and, more particularly, to a novel and highly-effective digital convergence correction process and apparatus in which substantial convergence correction is performed quickly and easily by unskilled users.
2. Description of the Prior Art
The image on the screen of a color television monitor is a composite of separate images (for example, red, green and blue images) that must be accurately superimposed or "converged" with respect to one another. Usually, a convergence correction signal is derived from analog processing of the horizontal and vertical deflection signals. Such conventional correction processes generally fail to produce sufficient accuracy. Especially in the case of tricolor projector-type television sets, the accuracy of convergence correction by conventional processes is insufficient, since even a slight misregister of the individual images noticeably degrades the composite image produced by such projector-type television sets. In cases where the television image is formed on a generally plane screen, the distance between the color television projector and various points on the screen varies as a function of displacement of the points from the optical axis of the projector. If the screen is large, e.g., about 50 inches or larger, or uneven, or if the television set has a high resolution (e.g., if the number of horizontal scan lines is twice that of conventional systems), the inaccuracy of the conventional convergence correction process has a particularly adverse effect on the quality of the image.
Digital techniques for correcting convergence have been developed in an attempt to eliminate the aforementioned defect in prior analog processes. One digital convergence correcting process is disclosed in Japanese Patent First Publication No. 58-215887, for example. The apparatus disclosed therein employs a test image or pattern, such as a dot pattern or cross-hatch pattern. The test pattern is reproduced on a television screen for convergence correction. Each dot or intersection of the test pattern is taken as an adjustment point. The correction needed at each adjustment point is derived as digital correction data. The corrections for all of the adjustment points for a given picture area are stored in a memory as digital correction data. The stored correction data for each adjustment point are read out synchronously with the beam scan on the screen. The digital correction data are converted into an analog signal indicative of the correction. The converted analog correction signal is supplied to a deflecting means that physically performs the convergence correction. This system allows the convergence to be corrected at each of the adjustment points.
In the foregoing conventional digital process for correcting convergence in a color television set, the number of adjustment points determines the accuracy of the adjustment. Thus to increase the adjustment accuracy it is necessary to increase the number of adjustment points and the number of separate adjustments. This naturally prolongs the time needed for adjustment. In addition, the adjustment is complex and requires the high level of skill found only in trained technicians. Of course, individual tastes may still be dissatisfied with the image on the screen despite the most accurate possible objective adjustment. For these reasons, it is desirable that some provision be made for easy adjustment of the color convergence by unskilled end users of the equipment.